An electrical generator converts mechanical energy to electrical energy. An electric motor does the opposite; it converts electrical energy to mechanical energy. More specifically, a voltage can be applied to drive an electric motor. When an electric actuator or motor is in motion, there is relative motion between the armature of the motor and a magnetic field in the motor. This relative motion generates a voltage within the motor that opposes the applied drive voltage. This motor-generated voltage is known as the Back-emf (BEMF), which has a magnitude proportional to the velocity of the motor's armature. This is a fundamental property of all actuators and is reliant on the same principles as a generator.
Because the BEMF is an inherent property of any actuator, the ability to measure it precisely allows for the use of a vast variety of control schemes. Controlling the BEMF and driving the actuator to the correct BEMF value allows for precise control of the actuator velocity and in turn the displacement of an associated physical element. Without such accuracy, problems in systems employing so controlled motors can occur. For example, if the BEMF estimate is higher than the true value, the actuator will be driven slower than desired. This can be undesirable in applications like cooling systems where running a fan too slow may cause the system to overheat. If the estimate is lower than the true value, the actuator will be driven too quickly. This can result in the actuator diaphragm hitting the wall of the casing in a speaker, for example, and causing distortion in output and/or damage to the actuator.
Various methods to track the BEMF are known. One example uses direct sensing of the BEMF, which systems require the actuator terminals to be tri-stated while measuring the BEMF. To make such a measurement, the motor driver needs to be cut off, which is undesirable in many cases and can cause a ringing in system that adds error and/or delay in taking the measurement. Moreover, such approaches also require the use of 3-Terminal motors.
Another example approach is to estimate the BEMF by measuring the voltage and/or current directed into the actuator. Such approaches have various drawbacks including requiring “sense resistors,” which reduce efficiency and take up area on the die or board supporting the electronics. This approach also requires complicated digital logic to derive the value of BEMF from the measured drive voltage current values and is more susceptible to variations in actuator parameters as well as variations due to temperature. Also, this approach depends on accurate and stable values for the voltage, current, and the DC resistance (Rdc) of the actuator. This is a non-trivial proposition because the Rdc of the motor varies with temperature and variables in production, and the accuracy of current and voltage measurements are limited in circuit implementations, largely due to non-idealities in the measurement circuitry and to interaction and noise coupling from the driver circuit. For example, the current measurements typically are accurate to 7-14% and voltage measurements to about 5-10%. This results in approximately 10 to 20% error in calculated BEMF values without taking temperature effects into account. Moreover, implementation of the BEMF calculations using the measured voltage and current add calculation errors that add up to the BEMF error because the calculations require floating point operations that are usually limited in accuracy by restrictions on speed and implementation of the digital logic due to area concerns.